This invention relates to a method and apparatus for digitally measuring strain or fatigue within a ferromagnetic material. More particularly, this invention relates to a method and apparatus which measures the change in time required for the magnetic domains within a ferromagnetic material to flip orientations in response to the application of a magnetic field to the ferromagnetic material as a strain or stress is applied to the ferromagnetic material. The invention may be used as a strain gage, attached to a substrate, in order to measure strain within the substrate.
In order to determine critical design parameters within various types of test elements or components within a machine, the designer must be able to accurately measure the stress (or strain) which the component may be subjected to under operating conditions. One method of measuring stress within a component is by the attachment of a strain gage to the component surface. Once calibrated, the strain gage can be used to determine the stresses within the component when the component is subjected to a load. Another method for non-destructive direct measurement of stress within a component made up of a ferromagnetic material, is based upon the attenuation of a magnetic pulse as it traverses the component.
Present strain gages, which are adapted to be attached to a substrate, operate on the principle that when a wire or foil is stretched the electrical resistance within the wire or foil changes due to the increase in length and decrease in diameter of the wire. Thus, by measuring the change in resistance of the wire or foil and referencing this change in resistance to a calibration of the strain gage, the stress or strain within the substrate onto which the strain gage is attached may be determined. The advantages of a resistance strain gage include relatively simple design and construction and easy attachment to the surface of any type of material. However, since the resistance within the wire or foil is also a function of the temperature to which the strain gage is subjected, as well as to other environment factors, the resistance type of strain gage is temperature limited and requires a complex functional algorithm in order to determine stress or strain within a material under varied or changing environments. Additionally, resistance strain gages provide a signal which is measured as a change in resistance, thus this signal is inherently analog in nature.
The other method of measuring stress, as disclosed within Japanese Pat. No. 56-10,527 issued to Y. Sougiyou, measures stress applied to a body by the attenuation of a magnetic flux within a detection piece made of a magnetic elastic material which is under stress. The stress is detected as a variation in the magnitude of the magnetic flux, which is measured by the amount of current produced within a detecting coil. This method of measuring stress also provides an analog output signal.